Conventional power conversion systems usually use power supply switching in constant voltage or constant current control mode. In a power conversion system, turning on or off of controlling switching elements on the primary winding of a transformer can periodically generate currents on the winding passing the switching elements, while the energy on the primary side is transmitted to the secondary side, and an AC current generated on the secondary windings is rectified and filtered passing through diodes and capacitors and converted into a DC voltage supplying to loads.
In start-up of a Flyback converter, because output voltage supplied to loads from the output terminal is far lower than the target voltage, a conventional feedback mechanisms adjusts the switch on the primary winding to operate under a maximum duty to give more power, incurring a large amount of inrush current that may exceed the capacity of each device on the primary side, bringing a high risk of damaging the switch and other devices. The existing technologies to prevent the large amount of inrush current are to add a soft start circuit on the power supply system to increase the start up time so that the voltage increases relatively smoothly. This can alleviate the inrush current to some extent, but it cannot eliminate the problem entirely. U.S. Patent Publication US20120274299 proposes to add a regulating circuit to adjust the PWM signal; however, the addition of the regulating circuit increases costs and makes the whole circuit more complex, and when it is under heavy load condition, each cycle will trigger the protection, as such this method is not applicable, and this system would produce sub-harmonic. It is within this context that embodiments of the present invention arise.